This book focuses on key findings and recent advances in diagnosis of intracranial arterial atherosclerotic stenosis(ICAS),with an emphasis on novel approaches to defining the underlying mechanisms of stroke in intracranial atherosclerotic disease(ICAD),in relation to possible treatment interventions.The importance of ICAS as a cause of ischemic stroke in whites is underestimated as compared to that of extracranial arterial atherosclerotic stenosis (ECAS) and nonvalvular atrial fibrillation (NVAF).On the other hand, intracranial arterial calcifications, stenosis and occlusions represent the most frequent abnormalities detected in intracranial arteries. Intracranial arterial stenosis is caused by an atherosclerotic plaque in more than 90% of cases (ICAS).Intracranial atherosclerotic stroke differs from extra¬cranial atherosclerotic stroke in many aspects, including risk factors and stroke patterns. Unlike in patients with ECAS or NVAF,stroke associated with ICAD occurs in association with various stroke mechanisms such as in situ thrombotic occlusion, artery-to-artery embolism, branch occlusion, and hemodynamic insufficiency.